As wireless communication systems such as cellular, satellite, and microwave communication systems become widely deployed and continue to attract a growing number of users, there is a continuing need to accommodate a large and variable number of communication subsystems transmitting a growing volume of data.
In many communication systems, frequency spectrum in wireless systems is divided by government or regulatory agencies to different frequency bands. These frequency bands may be assigned to certain operators or carriers. This arrangement may be used to minimize interference between different operators. However, the increasing need for additional bandwidth conflicts with the present fixed spectrum assignment approach. The fixed spectrum allocations do not adapt to actual usage, so if there is low use of part of the spectrum in an environment, the operators are not able to efficiently adapt the system usage to take advantage of that spectrum. Further, nonidealities of radio transmitters cause emission of unwanted distortion products in adjacent frequency channels. These unwanted out of band emissions may effectively deny access to those adjacent channels in addition to the frequency channel used for the transmission. However, since the power of such distortion products in adjacent channels is much lower than that of the communication signal in the main channel, the geographical area where access is denied on the adjacent channels (interference radius) is much smaller than the interference radius on the main channel. Whereas a conventional fixed network planning needs to leave a sufficient margin for the worst case that is contemplated, a system using “flexible spectrum usage” can possibly exploit such radio resources that are only available in a limited area, thereby achieving higher capacity for the overall radio system.
Present applications for communications in a wireless system are expanding the need for bandwidth, and hence, frequency spectrum. In addition to the moderate bandwidth requirements of voice communications from and to wireless telephones, the applications now supported include video, audio, and data exchanges that are much more demanding. Video streams sent to mobile devices such as PDAs, viewers, and cellphones require much more bandwidth than traditional voice communications. Further, often the communications are asymmetric; for example, a streaming video broadcast to many viewers over a radio frequency communications system may require much more downlink bandwidth (from a central base station to the users' receivers) than uplink bandwidth, so the spectrum usage needs to be allocated in a manner that reflects real time considerations.
In certain environments, additional bandwidth is frequently needed. In an apartment complex, office park, hospital or university, or in a densely populated urban neighborhood, the number of radio transceivers in a given area may be much denser than in a typical residential or rural area. These environments require additional flexibility. More radio capacity could be added by merely providing power, a network connection and an antenna to an additional base station. This might be considered analogous to adding a wireless LAN “hotspot” for internet access by computer users. However, in present wireless communications systems, adding additional base stations requires that the new equipment be in communication with a managing network, and that certain common reference parameters such as common timing references be known to the new base station and to neighboring base stations. Further the base stations using a particular frequency spectrum must be operated by a common operator so that the timing references used are known and understood amongst the base stations.
Presently, work is progressing on advancing the standards for wireless communications systems to support present and future enhanced services, such as providing capacity to replace wired telephony systems, support for video, audio or data (for example, software updates) broadcasts simultaneously to many users, support for data file communications, and support for internet services over an air interface, as well as support for existing voice, email, text, photographs, and SMS messaging services. The group of standards presently in development comprises the International Mobile Telecommunications (“IMT”) Advanced (“IMT-A”) project and the 3G long term evolution project, for example.
The third generation partnership project long term evolution (“3GPP LTE”) is the name generally used to describe an ongoing effort across the industry to improve the universal mobile telecommunications system (“UMTS”) for mobile communications. The improvements are being made to cope with continuing new requirements and the growing base of users. Goals of this broadly based project include improving communication efficiency, lowering costs, improving services, making use of new spectrum opportunities, and achieving better integration with other open standards and backwards compatibility with some existing infrastructure that is compliant with earlier standards. The project envisions a packet switched communications environment with support for such services as Voice over Internet Protocol (“VoIP”) and Multimedia Broadcast/Multicast Services (“MBMS”). MBMS may support services where base stations transmit to multiple user equipment simultaneously such as mobile television or radio broadcasts, for example. The 3GPP LTE project is not itself a standard-generating effort, but will result in new recommendations for standards for the UMTS.
The wireless communication systems as described herein are applicable to, for instance, IMT-A and 3GPP LTE compatible wireless communication systems and of interest is an aspect of LTE referred to as “evolved UMTS Terrestrial Radio Access Network,” or e-UTRAN. In general, e-UTRAN resources are assigned more or less temporarily by the network to one or more UEs by use of allocation tables, or more generally by use of a downlink resource assignment channel or physical downlink control channel (PDCCH). LTE is a packet-based system and, therefore, there may not be a dedicated connection reserved for communication between a UE and the network. Users are generally scheduled on a shared channel every transmission time interval (TTI) by a Node B or an evolved Node B (e-Node B). A Node B or an e-Node B controls the communications between user equipment terminals in a cell served by the Node B or e-Node B. In general, one Node B or e-Node B serves each cell. A Node B may be referred to as a “base station.” Resources needed for data transfer are assigned either as one time assignments or in a persistent/semi-static way. The LTE, also referred to as 3.9G, generally supports a large number of users per cell with quasi-instantaneous access to radio resources in the active state. It is a design requirement that at least 200 users per cell should be supported in the active state for spectrum allocations up to 5 megahertz (MHz), and at least 400 users for a higher spectrum allocation.
The UTRAN comprises multiple Radio Network Subsystems (RNSs), each of which contains at least one Radio Network Controller (RNC). However, it should be noted that the RNC may not be present in the actual implemented systems incorporating E-UTRAN. LTE may include a centralized or decentralized entity for control information. In UTRAN operation, each RNC may be connected to multiple Node Bs which are the UMTS counterparts to Global System for Mobile Communications (GSM) base stations. In E-UTRAN systems, the e-Node B may be, or is, connected directly to the access gateway (“aGW,” sometimes referred to as the services gateway “sGW”). Each Node B may be in radio contact with multiple UEs (generally, user equipment including mobile transceivers or cellphones, although other devices such as fixed cellular phones, mobile web browsers, laptops, PDAs, MP3 players, and gaming devices with transceivers may also be UEs) via the radio over the air or Uu interface.
In order to facilitate scheduling on the shared channel, the e-Node B transmits a resource allocation to a particular UE in a downlink-shared channel (PDCCH) to the UE. The allocation information may be related to both uplink and downlink channels. The allocation information may include information about which resource blocks in the frequency domain are allocated to the scheduled user(s), the modulation and coding schemes to use, what the size of the transport block is, and the like.
The lowest level of communication in the E-UTRAN system, Level 1, is implemented by the Physical Layer (“PHY”) in the UE and in the e-Node B. The PHY performs the physical transport of the packets between them over the air interface using radio frequency signals. In order to ensure a transmitted packet was received, an automatic retransmit request (“ARQ”) and a hybrid automatic retransmit request (“HARQ”) approach is provided. Thus, whenever the UE receives packets through one of several downlink channels, including command channels and shared channels, the UE performs a communications error check on the received packets, typically a Cyclic Redundancy Check (CRC), and in a later sub frame following the reception of the packets, transmits a response on the uplink to the e-Node B or base station. The response is either an Acknowledged (ACK) or a Not Acknowledged (NACK) message. If the response is a NACK, the e-Node B automatically retransmits the packets in a later sub frame on the downlink or DL. In the same manner, any UL transmission from the UE to the e-Node B is responded to, at a specific sub frame later in time, by a NACK/ACK message on the DL channel to complete the HARQ. In this manner, the packet communications system remains robust with a low latency time and fast turnaround time.
Current systems require a network communications system to allow base stations to coordinate with one another. Base stations sharing frequency spectrum typically use a common timing reference. Fixed frequency allocations controlled by a central agency ensure operators do not interfere with each other. However, these allocations may create underused and unused frequency spectrum.
A flexible spectrum usage (FSU) approach is envisioned as one solution to increasing the efficient use of frequency spectrum in evolving 3GPP LTE and IMT-A systems. In this approach, frequency spectrum may not be exclusively allocated in bands. These future systems will not rely on the fixed network planning and hardwired network backbone of the present approaches; instead, it is believed that these systems will use frequency spectrum in a manner that makes efficient allocations in real time or near real time, using agreed fairness protocols to share the spectrum between actors.
Wireless communications systems have an ongoing need for the capability to add additional bandwidth capacity or coverage in a manner similar to a wireless LAN hotspot approach, that is, by simply providing power and a network connection for another base station. In order to maintain the capability to support multiple operators and to provide simple hardware compatibility, additional capacity should be added without the requirement that the added equipment be in communication with a particular operator or other base stations in the geographic vicinity; instead, an ongoing need exists for methods and apparatuses to provide a system that will adapt to the presence of the added transceiver.